1. Field of the Invention
The present invention generally relates to precision coordinate measurement systems for positioning, pointing control and vibration cancellation and, more particularly, to a laser metrology system capable of providing real-time platform motion information at high precision and accuracy.
2. Description of Prior Art
Various applications require exacting coordinate control of equipment. For example, in the space industry there is a need for precision control of spacecraft and payloads. In the particular space context, liquid-filled inclinometers cannot be used due to their gravity and temperature dependence and limited resolution, and eddy current sensors are undesirable due to electromagnetic compatibility with the rest of the spacecraft.
Another application is astronomical inteferomtry. An astronomical interferometer is an array of controlled-position telescopes or mirror segments acting together to resolve at a higher resolution. Astronomical interferometers are widely used for optical astronomy, infrared astronomy, submillimetre astronomy and radio astronomy.
Optical (laser) metrology is an excellent Sensor option for the foregoing applications because it has excellent resolution. Laser metrology systems that can measure both absolute distance and displacement have broad applications ranging from coordinate measuring machines to deployable structures. Indeed, laser metrology systems have become a key component of space craft and, payload positioning systems, and of stellar interferometers where they are used to monitor path lengths and dimensions internal to the instrument.
As an example, the MSTAR sensor (Modulation Sideband Technology for Absolute Ranging) is an existing NASA system for measuring absolute distance, capable of resolving the integer cycle ambiguity of standard interferometers, and making it possible to measure distance with sub-nanometer accuracy. The design of this system is described by Lay et al., “MSTAR: An Absolute Metrology System With Submicrometer Accuracy”; New Frontiers in Stellar Interferometry, Proceedings of SPIE Volume 5491, p.1068 (October 2004). MSTAR is a general-purpose tool for conveniently measuring length to micron-absolute and sub-nm displacement, and has a wide range of possible applications.
Unfortunately, MSTAR and Other known laser metrology systems are relatively complicated, use more hardware (acousto-optic modulators, phase modulators, etc.), require stabilized lasers, and place tight tolerance limits on the optics and electronics. In addition, frequency sources must be calibrated and stabilized, and still there is a large degree of ambiguity of measurement.
Accordingly, it would be greatly advantageous to employ a swept frequency laser source and a calibrated reference cavity to measure unknown distance by comparison with the reference cavity. This approach would provide a far simpler laer metrology with reduced hardware overhead, reduced claibration and stabilization requirements, and reduced ambiguity of measurement.